1. Field of the Invention
This invention relates to a phase lock loop control system and more particularly to a system for rapidly bringing a variable output pulse signal into phase lock with a reference pulse signal in cases where the starting frequency of the output pulse signal is substantially different from the frequency of the reference pulse.
2. Description of the Prior Art
Because the present invention will be described hereinafter in connection with a phase lock motor speed control, the prior art with respect to such motor controls is pertinent. Phase lock speed control systems are employed in applications where the speed of the motor must be precisely correlated to the frequency of a series of clock pulses or like reference pulses that recur at regular intervals. Exemplary applications include video recorders, in which the speed of a read-write head must be synchronized with sync pulses from a horizontal oscillator, and data recording systems, in which the relative speed of a read-write head and a magnetic tape must be synchronized with a clock pulse generated within the system of which the motor that drives the read-write head is a part. The speed of the motor is changed by changing the frequency of the reference pulses; it is desirable to minimize the time required for the motor to change speed and achieve lock with the reference pulses.
Both the known prior art and the present invention employ a phase lock loop which drives a motor by means of a saturating amplifier until the motor approaches a speed that corresponds to a clock or reference pulse rate. One prior art technique for minimizing the time required for lock is to employ a low gain in the phase lock loop so that the motor can move into the linear range of operation without oscillation or like instabilities. Low loop gain is not satisfactory for precision speed control, however, because a substantial residual phase error exists.
Another prior art technique which avoids the problems of continuous low gain involves gain switching so that the system operates in a low gain mode as the motor approaches and enters the linear range and then switches to a high gain mode after lockup is achieved. Since gain is lower, the settling time is longer. Also, because of disturbances or transients injected when the system switches, time for the effect of the disturbances or transients to subside must be provided before a true locked condition results.